This invention relates to a polymer blend comprising polypropylene, an amount of a vinylaromatic/.alpha.-methylstyrene copolymer prepared by anionic polymerization at a temperature above the ceiling temperature of .alpha.-methylstyrene and, as a compatibilizer, an amount of an .alpha.-olefin copolymer rubber which has graft polymerized thereto an amount of a monovinylidene aromatic polymer. These polymer blends have very attractive combinations of properties, including thermoformability, moldability, resistance to naturally-occurring organic substances which tend to accelerate the degradation of some thermoplastic resin parts (e.g., fats and greases), ductility, low cost, and other characteristics. Such combinations of properties make these compositions useful for applications such as packaging, disposable containers and the like. The blends have many of the advantages of the constituent polymers and may be shaped into products by many of the known thermoplastic forming techniques, such as thermoforming, injection molding, stamping, forging, solid phase forming, rotary molding or the like.
Copolymers of a vinylaromatic monomer and .alpha.-methylstyrene prepared by anionic polymerization at a temperature above the ceiling temperature of .alpha.-methylstyrene (61.degree. C.) have been previously known in the art. A suitable process for their manufacture has been disclosed in EP No. 87,165. Because the most commonly available vinylaromatic monomer is styrene such copolymers are generically referred to as SAMS copolymers.
Such copolymers are very easily thermoformed and exhibit good impact strength and low temperature properties when thermoformed. In addition, SAMS copolymers have desirable high temperature properties. Because of their higher melt temperature, SAMS copolymers may be exposed to temperatures over 100.degree. C. without deformation.
Disadvantageously, however, SAMS copolymers have poor environmental stress crack resistance (ESCR). Environmental stress cracking occurs when molded resin parts are subjected to conditions where the part is both placed under stress and exposed to a fat- or grease-containing substance. These conditions occur commonly where materials such as grease- or fat-containing foods are packaged in a molded container. The presence of the grease or fat in the food in combination with the stress placed on the container, associated with filling, sealing and handling, cause such containers to become weakened and easily damaged.
Polypropylene, on the other hand, has relatively good resistance to the action of fats and greases and, therefore, is very desirable for the manufacture of containers for foods and similar materials. However, when it comes to manufacturing or forming techniques, polypropylene is much less versatile than SAMS copolymers. As is known, polypropylene, due to its relatively sharp melting point, is very difficult to thermoform.
While it might seem relatively straight forward to combine the two types of polymers to achieve a blend having desirable combinations of properties, the blending of these two polymers is complicated by the fact that the two resins are incompatible.
In general, it is known that it may be possible to form a blend of two otherwise incompatible resins by the use of a so-called compatibilizing agent. For examples of earlier attempts to blend olefin polymer and monovinylidene aromatic polymer resins see U.S. Pat. Nos. 4,386,187 and 4,386,188 to Grancio et al., the teachings of which are incorporated herein by reference, where a major amount of a crystalline olefin polymer is blended with a minor amount of an amorphous polymer and a block copolymer compatibilizer such as a styrene-butadiene-styrene block copolymer. See also European Patent Application Nos. 60,524 and 60,525, both filed Mar. 11, 1982, where styrene-butadiene block copolymers are used to compatibilize blends of an olefin polymer and a high impact polystyrene. In Japanese Patent Announcement Kokai No. 49-28627/1974, olefin and styrene-type resins are blended together with styrene-butadiene block copolymers. Japanese Patent Announcement Kokai No. 48-43031/1973 also teaches blends of a polyolefin, an aromatic vinyl polymer and a polymer selected from the group consisting of styrene-butadiene block copolymers, ethylene-vinyl acetate copolymers, ethylene-acrylic acid ester copolymers and ethylene-methacrylic acid ester copolymers. In European Patent Application No. 60,525 and U.S. Pat. No. 4,188,432, a hydrogenated styrene-butadiene-styrene block copolymer (i.e., styrene-ethylene-butylene-styrene type polymer) is blended with styrenic and olefinic polymers. Hydrogenated styrene-butadiene diblock copolymers (i.e., styrene-ethylene-butylene type polymers) have also been blended with amounts of olefinic and styrenic polymers. See, for example, British Pat. No. 1,363,466; U.S. Pat. No. 4,020,025; Japanese Pat. No. 81-38,338; German Pat. No. 241,375.
However, these general approaches to olefin/monovinylidene aromatic polymer blends provide insufficient property combinations and have other undesirable limitations, such as the amounts of the monovinylidene aromatic and olefinic polymers which can be blended. Moreover, previously known blends have failed to provide a composition having improved high temperature properties.